Piezoelectric/electrostrictive film element with a diaphragm having at least one stress releasing end section

ABSTRACT

A piezoelectric and/or electrostrictive film element including a ceramic substrate, and a piezoelectric or electrostrictive unit formed on the substrate and including a piezoelectric or electrostrictive layer between lower and upper electrodes. The substrate has a window closed by a diaphragm portion. The unit is disposed on the diaphragm portion such that at least one of the opposite ends of the unit is spaced apart from the edge of the window in a direction toward the center of the diaphragm portion. The end part of the diaphragm portion spaced apart from the edge of the window is upwardly convexed or downwardly concaved to provide a stress releasing section for effectively converting stresses generated in the piezoelectric or electrostrictive unit into displacement of the diaphragm portion. Also disclosed are methods for forming the stress releasing section by firing the unfired piezoelectric/electrostrictive layer formed on the diaphragm portion of the fired ceramic substrate.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a uni-morph, bi-morph or other type ofpiezoelectric and/or electrostrictive film element which generates ordetects displacement or force in the form of bending, deflection orflexure, and which can be used for actuators, filters, display devices,transformers, microphones, sounding bodies (such as loudspeakers),various resonators, oscillators, or vibrators, discriminators, gyros,sensors and other components and devices. The present invention is alsoconcerned with a method for producing such piezoelectric orelectrostrictive film elements. The term "element" used herein means anelement which is capable of transducing or converting an electric energyinto a mechanical energy, i.e., mechanical displacement, strain orvibrations, or converting a mechanical energy into an electric energy.

2. Discussion of the Related Art

In recent years, in the fields of optics and precision positioning ormachining operations, there has been an increasing demand for an elementwhose displacement is controlled for adjusting or controlling an opticalpath length or a position of a member or component of a device, on theorder of fractions of a micron (μm), and a detecting element adapted todetect infinitesimal displacement of a subject as an electric change. Tomeet the demand, there have been developed piezoelectric and/orelectrostrictive film elements (hereinafter referred to as "P/E filmelements") used for actuators or sensors, which elements comprise apiezoelectric material such as a ferroelectric material, and utilize thereverse or converse piezoelectric effect to produce a mechanicaldisplacement upon application of an electric field to the piezoelectricmaterial, or utilize the piezoelectric effect to produce an electricfield upon application of a pressure or mechanical stress. Among theseelements, a conventional uni-morph type P/E film element has beenfavorably used for a loudspeaker, for example.

There have been proposed ceramic P/E film elements used for variouspurposes, as disclosed in JP-A-3-128681 (i.e., in the co-pending U.S.patent applications Ser. Nos. 07/550,977, 07/860,128, 08/102,960,08/384,469, 08/392,083 and 08/452,092) and in JP-A-5-49270 (i.e., inU.S. Pat. No. 5,210,455 and U.S. patent application Ser. No.08/013,046), which were filed by the assignee of the present invention.One example of the disclosed elements has a ceramic substrate which hasat least one window, and is formed integrally with a thin diaphragmwhich closes the window or windows so as to provide at least onethin-walled diaphragm portion or vibratile portion. On an outer surfaceof each diaphragm portion of the ceramic substrate, there is formed apiezoelectric/electrostrictive unit (hereinafter referred to as "P/Eunit") which is an integral laminar structure consisting of a lowerelectrode, a piezoelectric/electrostrictive layer (hereinafter referredto as "P/E layer") and an upper electrode. The P/E unit is formed by asuitable film-forming method or process on the corresponding diaphragmportion of the ceramic substrate. The thus formed P/E film element isrelatively small-sized and inexpensive, and can be used as anelectromechanical transducer having high reliability. Further, thiselement has a high operating response, and provides a relatively largeamount of displacement by application of a low voltage, with arelatively large magnitude of force being generated. Thus, theabove-described element is favorably used as a member for an actuator,filter, display device, sensor or other components or devices.

To produce the P/E film element as described above, the lower electrode,P/E layer and upper electrode of each P/E unit are laminated in thisorder on the diaphragm portion of the ceramic substrate by a suitablefilm-forming method, and is subjected to heat treatment (firing orsintering) as needed, so that the P/E unit is formed integrally on thediaphragm portion. A further study of the inventors of the presentinvention revealed that the piezoelectric/electrostrictivecharacteristics of the element are deteriorated due to the heattreatment (firing or sintering) effected during the formation of the P/Eunit, more specifically, the P/E layer.

Described in detail, the P/E layer suffers from stresses due to firingshrinkage of the P/E layer or P/E unit which is in contact with thediaphragm portion of the ceramic substrate, during the heat treatment ofthe P/E layer. The stresses remain therein after the firing, and preventthe P/E film element from having a sufficiently dense sinteredstructure. In this case, the P/E film element can not exhibit desired orintended piezoelectric/electrostrictive characteristics. The residualstresses after the firing of the P/E layer undesirably deteriorate thepiezoelectric/electrostrictive characteristics, in particular, reducesan amount of displacement of the diaphragm portion generated uponactuation of the P/E unit.

To produce an actuator for a display device, for example, a plurality ofwindows are formed in a suitable pattern through a ceramic substrate,and P/E units as described above are formed on respective thin-walleddiaphragm portions that close the corresponding windows. In this case,the amounts of displacement of the diaphragm portions may beconsiderably reduced due to the stresses remaining in the P/E layerafter the firing thereof, particularly when two or more adjacent P/Eunits are actuated at the same time, as compared with the displacementamount where a single P/E unit is actuated. That is, when the twoadjacent P/E units are actuated simultaneously, for example, thedisplacement of the diaphragm portion which carries one of the two P/Eunits interferes with that of the diaphragm portion which carries theother P/E unit, resulting in reduction of the amounts of displacement ofthese diaphragm portions. The reduction in the amounts of displacementof the diaphragm portions is serious where the spacing between theadjacent P/E units is reduced to meet a recent demand for a largernumber of the P/E units per unit area.

SUMMARY OF THE INVENTION

It is therefore a first object of the present invention to provide apiezoelectric/electrostrictive film element in which eachpiezoelectric/electrostrictive unit is formed by a film-forming methodon an outer surface of a thin-walled diaphragm portion of a ceramicsubstrate, and which piezoelectric/electrostrictive film element canefficiently convert stresses generated in the P/E unit into a largeamount of displacement upon application of a relatively low voltage, anddoes not suffer from considerable reduction in the amount ofdisplacement where two or more P/E units formed on respective diaphragmportions are actuated at the same time.

It is a second object of the invention to provide a method suitable forproducing a piezoelectric/electrostrictive film element capable ofexhibiting such excellent characteristics as described above.

The above-indicated first object may be attained according to a firstaspect of the present invention which provides apiezoelectric/electrostrictive film element comprising: (a) a ceramicsubstrate including a base portion having at least one window, and adiaphragm portion formed as an integral part of the base portion andclosing each of the at least one window; (b) a film-likepiezoelectric/electrostrictive unit including a lower electrode, apiezoelectric/electrostrictive layer and an upper electrode, which areformed in the order of description on an outer surface of the diaphragmportion by a film-forming process, the piezoelectric/electrostrictiveunit being disposed on the diaphragm portion such that at least one ofopposite end faces of the piezoelectric/electrostrictive unit is spacedapart from a corresponding one of opposite portions of a periphery ofeach window in a direction toward a center of the diaphragm portion; and(c) a stress releasing section constituted by each of at least one ofopposite end parts of the diaphragm portion which is located betweensaid at least one of the opposite end faces of thepiezoelectric/electrostrictive unit and the corresponding at least oneof the opposite portions of the periphery of each window, the stressreleasing section being curved with respect to a plane which includes amajor surface of the base portion at which each window is closed by thediaphragm portion.

In the piezoelectric/electrostrictive film element constructed accordingto the present invention, the stress releasing section is formed at atleast one of opposite end parts of the diaphragm portion of the ceramicsubstrate on which the piezoelectric/electrostrictive unit is notdisposed. In the presence of the stress releasing section, thepiezoelectric/electrostrictive unit is less likely to be influenced by aforce received from the ceramic substrate, whereby the stressesgenerated in the piezoelectric/electrostrictive unit can be convertedinto displacement with high efficiency, while assuring a large amount ofdisplacement of the diaphragm portion, upon application of a relativelylow voltage to the piezoelectric/electrostrictive unit.

Further, where the film element is provided with a plurality ofpiezoelectric/electrostrictive units, the amounts of displacement of thepiezoelectric/electrostrictive units are not substantially reduced evenwhen these units are simultaneously actuated, as compared with theamount of displacement when each piezoelectric/electrostrictive unit isactuated alone. Thus, the present film element assures a substantiallyconstant amount of displacement of each diaphragm portion, andsubstantially consistent operating characteristics of the film element,irrespective of the number of the piezoelectric/electrostrictive unitswhich are simultaneously actuated.

According to a first preferred arrangement of the first aspect of thepresent invention, the opposite portions of the periphery of each windoware opposed to each other in a direction parallel to a straight linewhich is parallel to the plane of the major surface of the base portionand passes a center of each window and along which each window has ashortest dimension. In this case, the dimension of the stress releasingsection as measured in the direction is not greater than 40% of theshortest dimension of each window.

According to a second preferred arrangement of the first aspect of theinvention, the diaphragm portion has at least one inflection point.

According to a third preferred arrangement of the first aspect of theinvention, the stress releasing section includes an end part adjacent tothe corresponding one of the opposite portions of the periphery of eachwindow, the end part being curved protruding in a direction away fromthe plane of the major surface of the base portion.

According to a fourth preferred arrangement of the first aspect of theinvention, the piezoelectric/electrostrictive unit is disposed such thatboth of the opposite end faces of the piezoelectric/electrostrictiveunit are spaced apart from the corresponding opposite portions of theperiphery of each window in the direction toward the center of thediaphragm portion. In this case, the stress releasing section isprovided at each of the opposite end parts of the diaphragm portionlocated between the opposite end faces of thepiezoelectric/electrostrictive unit and the corresponding oppositeportions of the periphery of each window.

According to a fifth preferred arrangement of the first aspect of theinvention, the piezoelectric/electrostrictive unit is disposed such thatonly one of the opposite end faces of the piezoelectric/electrostrictiveunit is spaced apart from the corresponding one of the opposite portionsof the periphery of each window in the direction toward the center ofthe diaphragm portion. In this case, the stress releasing section isprovided at only one of the opposite end parts of the diaphragm portionlocated between the one of the opposite end faces of thepiezoelectric/electrostrictive unit and the corresponding one of theopposite portions of the periphery of each window.

According to a sixth preferred arrangement of the first aspect of theinvention, the stress releasing section has an upwardly convex shape,protruding from the plane in a direction away from each window.

According to one advantageous feature of the fourth preferredarrangement of the first aspect of the invention, the stress releasingsection provided at each of the opposite end parts of the diaphragmportion has an upwardly convex shape, protruding from the plane in adirection away from each window, the diaphragm portion including acentral part on which the piezoelectric/electrostrictive unit is formed,the central part having a downwardly concave shape, protruding from theplane in a direction toward each window.

According to one advantageous feature of the fifth preferred arrangementof the first aspect of the invention, the stress releasing sectionprovided at only one of the opposite end parts of the diaphragm portionhas an upwardly convex shape, protruding from the plane in a directionaway from each window, a part of the diaphragm portion which includesthe other of the opposite end parts and on which thepiezoelectric/electrostrictive unit is formed having a downwardlyconcave shape, protruding from the plane in a direction toward eachwindow.

According one advantageous feature of the sixth preferred arrangement ofthe first aspect of the invention, the ceramic substrate comprises adiaphragm plate which includes the diaphragm portion and which is formedon the major surface of the base portion such that the diaphragm portioncloses each window. In this case, the upwardly convex shape of thestress releasing section has a height not greater than twice a thicknessof the diaphragm portion. The height is measured from one of oppositemajor surfaces of the diaphragm plate which is remote from the baseportion.

According to a seventh preferred arrangement of the first aspect of theinvention, the stress releasing section has a downwardly concave shape,protruding in a direction toward each window.

According to one advantageous feature of the seventh preferredarrangement of the first aspect of the invention, the stress releasingsection is provided on at least one of opposite sides of thepiezoelectric/electrostrictive unit.

According to another advantageous feature of the seventh preferredarrangement of the first aspect of the invention, the ceramic substratecomprises a diaphragm plate which includes the diaphragm portion andwhich is formed on the major surface of the base portion such that thediaphragm portion closes each window. The downwardly concave shape ofthe stress releasing section has a height not greater than twice athickness of the diaphragm portion. The height is measured from one ofopposite major surfaces of the diaphragm plate which is remote from thebase portion.

In the piezoelectric/electrostrictive film element constructed accordingto the present invention, the stress releasing section may have acorrugated shape having at least one upwardly convex part and at leastone downwardly concave part. The diaphragm portion may have an averagecrystal grain size of not greater than 5 μm, and a thickness of notgreater than 30 μm. The piezoelectric/electrostrictive unit may have athickness of not greater than 100 μm.

The above-indicated second object of the invention may be attainedaccording to a second aspect of the invention which provides a method ofproducing a piezoelectric/electrostrictive film element comprising: aceramic substrate including a base portion having at least one window,and a diaphragm portion formed as an integral part of the base portionand closing each of the at least one window; and a film-likepiezoelectric/electrostrictive unit including a lower electrode, apiezoelectric/electrostrictive layer and an upper electrode, which areformed in the order of description on an outer surface of the diaphragmportion by a film-forming process, the method comprising the steps of:preparing the ceramic substrate in which the diaphragm portion has anupwardly convex shape protruding in a direction away from each window;forming, by a film-forming process, at least the lower electrode and thepiezoelectric/electrostrictive layer of thepiezoelectric/electrostrictive unit, on the upwardly convex outersurface of the diaphragm portion such that at least one of opposite endfaces of the unit is spaced apart from a corresponding one of oppositeportions of a periphery of each window in a direction toward a center ofthe diaphragm portion; and firing the piezoelectric/electrostrictivelayer such that a part of the diaphragm portion on which thepiezoelectric/electrostrictive unit is formed is downwardly curved so asto have a downwardly concave shape protruding in a direction toward eachwindow, while the at least one of opposite end parts of the diaphragmportion is upwardly curved so as to have an upwardly convex shapeprotruding in a direction away from each window, each of the at leastone of opposite end parts of the diaphragm portion providing a stressreleasing section.

The method of producing the piezoelectric/electrostrictive film elementaccording to the present invention permits commercially advantageousmanufacture of the film elements with improved efficiency.

According to a first preferred arrangement of the second aspect of theinvention, the step of forming at least the lower electrode and thepiezoelectric/electrostrictive layer comprises forming the lowerelectrode and the piezoelectric/electrostrictive layer such that both ofthe opposite end faces of the piezoelectric/electrostrictive unit arespaced apart from the corresponding opposite portions of the peripheryof each window in the direction toward the center of the diaphragmportion, the step of firing the piezoelectric/electrostrictive layercomprising firing the piezoelectric/electrostrictive layer such thatboth of the opposite end parts of the diaphragm portion are upwardlycurved so that each of the opposite end parts provides the stressreleasing section, while a central part of the diaphragm portion betweenthe opposite end parts is downwardly curved.

According to a second preferred arrangement of the second aspect of theinvention, the step of forming at least the lower electrode and thepiezoelectric/electrostrictive layer comprises forming the lowerelectrode and the piezoelectric/electrostrictive layer such that onlyone of the opposite end faces of the piezoelectric/electrostrictive unitis spaced apart from the corresponding one of the opposite portions ofthe periphery of each window in the direction toward the center of thediaphragm portion, the step of firing the piezoelectric/electrostrictivelayer comprising firing the piezoelectric/electrostrictive layer suchthat only one of the opposite end parts of the diaphragm portion isupwardly curved so as to provide the stress releasing section, while apart of the diaphragm portion which includes the other of the oppositeend parts and on which the piezoelectric/electrostrictive unit is formedis downwardly curved.

The above-indicated second object of the invention may be attainedaccording to a third aspect of the invention which provides a method ofproducing a piezoelectric/electrostrictive film element comprising: aceramic substrate including a base portion having at least one window,and a diaphragm portion formed as an integral part of the base portionand closing each of the at least one window; and a film-likepiezoelectric/electrostrictive unit including a lower electrode, apiezoelectric/electrostrictive layer and an upper electrode, which areformed in the order of description on an outer surface of the diaphragmportion by a film-forming process, the method comprising the steps of:preparing the ceramic substrate in which the diaphragm portion iscorrugated and has a plurality of upwardly convex parts in a directionaway from each window and a plurality of downwardly concave partsprotruding in a direction toward each window, the convex parts and theconcave parts being arranged alternately; forming, by a film-formingprocess, at least the lower electrode and thepiezoelectric/electrostrictive layer of thepiezoelectric/electrostrictive unit, on the corrugated outer surface ofthe diaphragm portion such that at least one of opposite end faces ofthe unit is spaced apart from a corresponding one of opposite portionsof a periphery of each window in a direction toward a center of thediaphragm portion; and firing the piezoelectric/electrostrictive layersuch that a part of the diaphragm portion on which thepiezoelectric/electrostrictive unit is formed is downwardly curved so asto have a downwardly concave shape protruding in the direction towardeach window, while the at least one of opposite end parts of thediaphragm portion remains corrugated, each of the at least one ofopposite end parts of the diaphragm portion providing a stress releasingsection.

The above-indicated second object of the invention may be attainedaccording to a fourth aspect of the invention which provides a method ofproducing a piezoelectric/electrostrictive film element comprising: aceramic substrate including a base portion having at least one window,and a diaphragm portion formed as an integral part of the base portionand closing each of the at least one window; and a film-likepiezoelectric/electrostrictive unit including a lower electrode, apiezoelectric/electrostrictive layer and an upper electrode, which areformed in the order of description on an outer surface of the diaphragmportion by a film-forming process, the method comprising the steps of:preparing the ceramic substrate in which each of opposite end parts ofthe diaphragm portion which are adjacent to opposite portions of aperiphery of each window is downwardly curved so as to have a downwardlyconcave shape protruding in a direction toward each window, while acenter part of the diaphragm portion between the opposite end parts isupwardly curved so as to have an upwardly convex shape protruding in adirection away from each window; forming, by a film-forming process, atleast the lower electrode and the piezoelectric/electrostrictive layerof the piezoelectric/electrostrictive unit, on an outer surface of theupwardly curved center part of the diaphragm portion; and firing atleast the piezoelectric/electrostrictive layer, so that the opposite endparts of the diaphragm portion each having the downwardly concave shapeprovide two stress damping sections.

According to one preferred arrangement of the fourth aspect of theinvention, the step of preparing the ceramic substrate comprises formingthe diaphragm portion such that the diaphragm portion generally has anupwardly convex shape protruding in the direction away from each window,and pressing a jig against at least a center part of the diaphragmportion in the direction toward each window.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and optional objects, features and advantages of the presentinvention will be better understood by reading the following detaileddescription of presently preferred embodiments of the invention, whenconsidered in connection with the accompanying drawings, in which:

FIG. 1 is an exploded perspective view showing one example of a basicstructure of a piezoelectric/electrostrictive film element constructedaccording to the present invention;

FIG. 2 is an elevational view in cross section of thepiezoelectric/electrostrictive film element of FIG. 1;

FIG. 3 is an enlarged view in cross section of the film element of FIG.1, which is taken in a plane which is parallel to the short side of awindow and which passes the center of the window;

FIG. 4 is a view corresponding to that of FIG. 3, showing anotherexample of the piezoelectric/electrostrictive film element according, tothe present invention;

FIG. 5 is a view corresponding to that of FIG. 3, showing still anotherexample of the piezoelectric/electrostrictive film element according tothe present invention;

FIG. 6 is a view corresponding to that of FIG. 3, showing a furtherexample of the piezoelectric/electrostrictive film element according tothe present invention;

FIG. 7 is a view corresponding to that of FIG. 3, showing a stillfurther example of the piezoelectric/electrostrictive film elementaccording to the present invention;

FIGS. 8(a)-8(c) are enlarged views partly in cross section, showingprocess steps of a method of producing thepiezoelectric/electrostrictive film element of FIG. 3;

FIGS. 9(a)-9(c) are enlarged views partly in cross section, showingprocess steps of a method of producing thepiezoelectric/electrostrictive film element of FIG. 4;

FIGS. 10(a)-10(c) are enlarged views partly in cross section, showingprocess steps of a method of producing thepiezoelectric/electrostrictive film element of FIG. 5;

FIGS. 11(a)-11(c) are enlarged views partly in cross section, showingprocess steps of a method of producing thepiezoelectric/electrostrictive film elements of FIGS. 6 and 7;

FIGS. 12(a) and 12(b) are enlarged views partly in cross section,showing process steps for producing a ceramic substrate used in themethod of FIG. 11;

FIG. 13 is an exploded perspective view showing another example of thepiezoelectric/electrostrictive film element of the invention having aplurality of piezoelectric/electrostrictive units;

FIG. 14 is a cross sectional view taken along line 14--14 of FIG. 13;and

FIG. 15 is a cross sectional view corresponding to that of FIG. 14,showing still another example of the piezoelectric/electrostrictive filmelement of the invention having a plurality ofpiezoelectric/electrostrictive units.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

There will be hereinafter described one embodiment of apiezoelectric/electrostrictive film element according to the presentinvention, wherein a piezoelectric/electrostrictive unit is formed by afilm-forming method or process on an outer surface of a diaphragmportion which closes a window of a ceramic substrate, and wherein asuitably dimensioned stress releasing or damping section is formed on atleast one section of the diaphragm portion on which thepiezoelectric/electrostrictive unit is not disposed. In this embodiment,the ceramic substrate has one window.

Referring to FIGS. 1 and 2 showing one example of apiezoelectric/electrostrictive film element (hereinafter referred to as"P/E film element") according to the present invention, a ceramicsubstrate 2 has an integral structure which consists of a base plate 4having a predetermined thickness, the base plate 4 having a rectangularwindow 6 of a suitable size, and a relatively thin diaphragm plate 8which closes the window 6. The diaphragm plate 8 is superposed on one ofthe opposite major surfaces of the base plate 4 which serves as asupport member. The diaphragm plate 8 has a diaphragm portion 10 whichcorresponds to the window 6 of the base plate 4. On the outer surface ofthe diaphragm portion 10 of the planar ceramic substrate 2, a lowerelectrode film 12, a piezoelectric/electrostrictive layer (hereinafterreferred to as "P/E layer") 14 and an upper electrode film 16 arelaminated in this order by a known film-forming method, so as to form afilm-like piezoelectric/electrostrictive unit (hereinafter referred toas "P/E unit) ". As known in the art, a suitable voltage is applied tothe lower and upper electrodes 12, 16, through respective lead portions(not shown).

Where the P/E film element constructed as described above is used as anactuator, a voltage is applied between the two electrodes 12, 16 of theP/E unit 18 in a known manner, so that the P/E layer 14 is exposed to anelectric field, and undergoes a mechanical distortion induced by theelectric field. Consequently, the P/E unit 18 causes a flexing, bendingor deflecting displacement or force due to the transverse effect ofdistortion of the P/E layer 14, such that the displacement or force actson the ceramic substrate 2 (diaphragm portion 10) in a directionperpendicular to the plane or major surfaces of the substrate 2.

The P/E film element according to the present invention has a suitablestress releasing section which is formed as part of the diaphragmportion 10 of the ceramic substrate 2 at which the P/E unit 18 is notformed. Described more specifically, in the present P/E film element, atleast one of the opposite end faces of the P/E unit 18 (as seen in FIG.2) formed on the diaphragm portion 10 is located inwardly of thediaphragm portion 10 so that the end face of the P/E unit 18 is inwardlyoffset or spaced apart from the corresponding end of the window 6 formedin the ceramic substrate 2. In other words, the stress releasing sectionconsists of a part of the diaphragm portion which is located between theat least one of the opposite end faces of the P/E unit 18 and thecorresponding end or ends of the window 6. In a P/E film element asshown in FIG. 3, for instance, two stress releasing sections 20, 20 areformed on the opposite sides (as seen in FIG. 3) of the P/E unit 18, atthe sections of the diaphragm portion 10 located between the oppositeend faces of the P/E unit 18 and the corresponding opposite ends of thewindow 6. In other words, the two stress releasing sections 20, 20consist of the above-indicated sections of the diaphragm portion 10which have respective width dimensions n, n' as indicated in FIG. 3 andwhich do not carry the P/E unit 18 thereon. As shown in FIG. 3, each ofthe stress releasing sections 20 has an upwardly curved convex shapewhich protrudes outwards in a direction away from the bottom of thewindow 6. In the presence of the thus formed stress releasing sections20 of the diaphragm portion 10, stresses arising in the P/E unit 18 maybe converted into displacement with high efficiency, and the diaphragmportion 10 undergoes a sufficiently large amount of displacement byapplication of a relatively low voltage to the P/E unit 18. In addition,the provision of the stress releasing sections 20 as described above iseffective to prevent reduction in the amount of displacement of thediaphragm portion 10 when two or more adjacent P/E units (18) aresimultaneously actuated. Thus, the stress releasing sections 20 make itpossible to satisfy the demand for increased density of the P/E units 18per unit area of the P/E film element.

Upon actuation of the P/E unit 18 formed on the diaphragm portion 10having the two stress releasing sections 20, 20 which are formed on theopposite sides of the P/E unit, respectively and which have the upwardlycurved convex shape as shown in FIG. 3, the diaphragm portion 10 isdisplaced downward as indicated by an arrow in FIG. 3, in other words,is deflected toward the window 6. In the presence of the stressreleasing sections 20, 20 formed as described above, the stressesgenerated in the P/E unit 18 upon actuation thereof are less likely tobe influenced by a force from a base portion of the ceramic substrate 2(i.e., base plate 4), whereby the stresses generated in the P/E unit 18are converted into displacement of the diaphragm portion 10 with highefficiency. Further, the diaphragm portion 10 undergoes a sufficientlylarge amount of displacement by application of a relatively low voltage.The stress releasing sections 20 are effective to avoid or reduce theinfluence of the force from the base portion of the ceramic substrate 2which would act on the P/E unit 18 upon actuation thereof. Accordingly,the amount of displacement of each diaphragm portion 10 is less likelyto be reduced even when the two or more adjacent P/E units 18 areactuated at the same time, and the amount of displacement of thesimultaneously actuated P/E units 18 is not so different from thedisplacement amount when each P/E unit 18 is actuated independently ofthe other P/E units.

The P/E film element of FIG. 3 has the diaphragm portion 10 whosecentral part carrying the P/E unit 18 is downwardly curved in a concaveshape, protruding into the window 6, so that the diaphragm portion 10generally assumes an "M" shape in cross section. In particular, thepresent P/E film element having the "M"-shaped diaphragm portion 10 withthe two stress releasing sections 20, 20 which are formed on theopposite sides of the P/E unit 18 effectively yields the advantages asdescribed above. The term "end face" of the P/E unit 18 generally meansthe end faces of the three layers 12, 14, 16 of the P/E unit 18, whichare flush with one another. Where the P/E layer 14 and the upperelectrode 16 are longer than the lower electrode 12 as indicated by abroken line of FIG. 3, for example, the end face of the P/E unit 18 isdefined as the end face of the lower electrode 12. Thus, the widthdimension n, n' of each stress releasing section 20 is determined by theend face of the shortest one of the three layers of the P/E unit 18.

It is to be understood that the construction of the P/E film elementaccording to the present invention is by no means limited to that of thefilm element of FIG. 3. For instance, the two stress releasing sections20, 20 need not be symmetrically located on the opposite sides of theP/E unit 18. The principle of the present invention may be embodied inthe form of P/E film elements as shown in FIG. 4 through FIG. 7, whichprovide substantially the same advantages as described above.

In the P/E film element shown in FIG. 4, the P/E unit 18 is formed onthe diaphragm portion 10 such that the P/E unit 18 is offset from thecentral part of the diaphragm portion 10 toward one of the opposite endsof the window 6, thus providing only one stress releasing section 20 onthe side of the end face of the P/E unit 18 which is remote from theabove-indicated one end of the window 6. Namely, the stress releasingsection 20 consists of a part of the diaphragm portion 10 which islocated between the above-indicated end face of the P/E unit 18 and thecorresponding end of the window 6. As shown in FIG. 4, the stressreleasing section 20 has an upwardly curved convex shape which protrudesoutwards, in a direction away from the window 6. On the other hand, theother part of the diaphragm portion 10 on which the P/E unit 18 isformed has a downwardly curved concave shape which protrudes inwardstoward the bottom of the window 6. In the present P/E film elementwherein the diaphragm portion 10 consists of the upwardly curved convexpart and the downwardly curved concave part which are contiguous to eachother, the convex part function as the stress releasing section 20 whichimproves piezoelectric/electrostrictive characteristics when the two ormore adjacent P/E units 18 are simultaneously actuated. Further, theconcave part of the diaphragm portion 10 which bears the P/E unit 18 iseffective to reduce the rigidity of the diaphragm portion 10 as a wholein the direction of its displacement upon application of the drivevoltage to the P/E unit 18. Thus, the present P/E film element iscapable of assuring a sufficiently large amount of displacement. In thefilm element shown in FIG. 4, the end face of the P/E unit 18 which isremote from the stress releasing section 20 is located at thecorresponding end of the concave part of the diaphragm portion 10.However, the end face of the P/E unit 18 may extend beyond thecorresponding end of the window 6, i.e., may extend over the base plate4, as indicated by a one-dot chain line in FIG. 4.

Referring next to FIG. 5, there will be described another form of theP/E film element of the present invention. The film element of FIG. 5 isidentical with that of FIG. 3 in which the two stress releasing sections20, 20 are formed on the opposite sides of the P/E unit 18,respectively. Unlike the stress releasing sections 20 of FIG. 3 each ofwhich is constituted by a single upwardly curved convex part, each ofthe stress releasing sections 20 in the P/E film element of FIG. 5 has acorrugated shape in which the convex and concave parts are contiguous toeach other. The thus formed stress releasing sections 20 are capable ofassuring similar advantages as described above. Although the two stressreleasing sections 20, 20 are provided on the opposite sides of the P/Eunit 18 in the film element of FIG. 5, a single corrugated stressreleasing section 20 may be provided on one of the opposite sides of theP/E unit 18, as in the film element of FIG. 4.

Referring to FIGS. 6 and 7, there are shown other forms of the P/E filmelement constructed according to the present invention. In the filmelements of the preceding examples of FIGS. 3-5, the end part of thestress releasing section 20 adjacent to the corresponding end of thewindow 6 protrudes outwards in the direction away from the window 6. Incontrast, the corresponding end part of the stress releasing section 20of FIGS. 6 and 7 protrudes inwards toward the bottom of the window 6.

In the P/E film element of FIG. 6, the central part of the diaphragmportion 10 which carries the P/E unit 18 is downwardly curved on theside of the window 6, while, in the film element of FIG. 7, the centralpart of the diaphragm portion 10 which carries the P/E unit 18 isupwardly curved and protrudes outwards in the direction away from thewindow 6. In particular, the construction of the P/E film element ofFIG. 6 is effective to reduce the rigidity of the diaphragm portion 10in the direction of displacement of the diaphragm portion 10 uponapplication of a voltage thereto, assuring a large amount ofdisplacement thereof.

In the P/E film elements of FIGS. 6 and 7, the two concave stressreleasing sections 20, 20 are formed on the opposite sides of the P/Eunit 18, respectively. However, the film elements of FIGS. 6 and 7 maybe modified such that a single stress releasing section 20 is formed onone of the opposite sides of the P/E unit 18, as in the film element ofFIG. 4.

It will be understood from the above description that the end part ofthe stress releasing section 20 adjacent to the corresponding end of thewindow 6 preferably constitute a part of the convex or concave shape.Namely, it is desirable that the stress releasing section 20 does nothave a flat end part which is flush with the upper surface of theceramic substrate 2 and which extends straight from the peripheral edgeof the window 6. The flat or straight end part of the stress releasingsection 20 would deteriorate its stress releasing effect, in otherwords, reduce the amount of displacement of the P/E units 18 when theyare simultaneously actuated. Since such a flat end part would notexhibit a sufficiently high stress releasing effect, the stressreleasing section constituted by a combination of the flat part andcurved part is disadvantageous where the density of the P/E units 18 perunit area of the film element is high. It is preferable that the widthdimension (n, n') of each stress releasing section located between theend face of the P/E unit and the corresponding end of the window be assmall as possible, while assuring a sufficient stress releasing effect.

As is apparent from the above description, the diaphragm portion 10having the stress releasing section(s) 20 constructed according to thepresent invention preferably has at least one inflection point forattaining an enhanced stress releasing effect. Namely, the diaphragmportion 10 formed according to the present invention preferably has atleast one convex portion and at least one concave portion which arecontiguous to each other, so as to provide at least one inflection pointin the diaphragm portion 10. It is noted that the inflection point maybe either in the stress releasing section 20 or in the portion of thediaphragm portion 10 which carries the P/E unit 18 thereon.

The stress releasing section 20 provided as part of the diaphragmportion 10 according to the present invention exhibits the desiredstress releasing effect as described above as long as the stressreleasing section 20 is formed along the peripheral edges of the window6 of the ceramic substrate 2. More advantageously, the stress releasingsection 20 is formed as a peripheral part of the diaphragm portion 10over a width n, n' not greater than 40% of the short side dimension (m)of the rectangular window 6, as measured along a straight line which isparallel to the short sides of the rectangle of the window 6 and whichpasses the center of the window 6. The dimension n, n' is preferably notgreater than 30%, and more preferably, not greater than 15% of thedimension (m). It is noted that the width dimensions n and n' are notnecessarily identical with each other.

In view of the displacement characteristics of the P/E film element, itis preferable that the stress releasing section 20 constructed accordingto the present invention have an upwardly curved convex part, or adownwardly curved concave part. The maximum height (indicated by "a" inFIGS. 3 and 4) of the convex part of the stress releasing section 20 andthe maximum depth (indicated by "d" in FIGS. 6 and 7) of the concavepart of the stress releasing section 20 as measured from the uppersurface of the substrate 2 are favorably determined so as not to exceedtwice the thickness of the diaphragm portion 10. In particular, themaximum height "a" and the maximum depth "d" are determined to be notsmaller than 1 μm and not larger than the thickness of the diaphragmportion 10.

The ceramic substrate 2 which carries the P/E unit 18 thereon is made ofa known ceramic material, and is favorably selected from stabilizedzirconia, partially stabilized zirconia, alumina and mixtures thereof.Particularly favorably used is a material as disclosed by the presentinventors in U.S. Pat. No. 5,430,344, which contains as a majorcomponent zirconia which is partially stabilized by adding acompound(s), such as yttrium oxide, and which has a crystal phase thatconsists essentially of a tetragonal phase or mixture of at least twokinds of cubic, tetragonal and monoclinic phases. The ceramic substrate2 made of the above-described material exhibits high mechanical strengthand high toughness even with a small thickness, and is less likely tochemically react with the piezoelectric/electrostrictive material. Theceramic substrate 2 is preferably produced by 1) preparing a green sheetwhich gives the base plate 4 and which is formed with an aperture(window 6) by use of a metal mold or by ultrasonic machining ormechanical machining, 2) superposing a thin green sheet which gives thediaphragm plate 8 (diaphragm portion 10) on the green sheet for the baseplate 4 and bonding the green sheets together by thermo compression, and3) firing the green sheets into an integral structure. The ceramicsubstrate 2 thus obtained exhibits high reliability. To assuresufficiently high mechanical strength, the average crystal grain size ofthe ceramic material for the diaphragm portion 10 which carries the P/Eunit 18 thereon is generally controlled to be not greater than 5 μm,preferably, not greater than 2 μm, more preferably, not greater than 1μm. To assure a sufficiently high operating response speed of theelement and large displacement of the element, the thickness of thediaphragm portion 10 is generally 50 μm or smaller, preferably, in arange of 1 μm-30 μm, more preferably in a range of 3 μm-15 μm.

Each of the green sheets for the base plate 4 and diaphragm plate 8 mayconsist of a plurality of thin sheets which are superposed on eachother. While the window 6 of the ceramic substrate 2 or the diaphragmportion 10 has a rectangular shape in the present embodiments, the shapeof the window 6 may be suitably selected from other shapes, such as acircular, polygonal and elliptical shape, and combinations of theseshapes, depending upon the application or utility of the P/E filmelement. In the embodiments of FIGS. 3-7 wherein the window 6 has arectangular shape as shown in FIG. 1, the above-indicated dimension (m)is the length of the short sides of the window 6. Where the window 6 hasa circular or elliptical shape, for example, the diameter of the circleor the length of the minor axis of the ellipse is the shortest dimensionof the circular or elliptical window 6, as measured along a straightline which passes the center of the window. Thus, (m) represents theshortest dimension of the window 6 of any shape as measured along thestraight line passing the center of the window. In the presentinvention, the stress releasing sections 20 are preferably formed alongthe respective opposite portions of the periphery of the window 6 whichgive the shortest dimension (m), so that the stress releasing sectionsprovide a higher degree of stress releasing effect.

The above-indicated electrodes 12, 16 and the P/E layer 14 are formed bya suitable film-forming method on the diaphragm portion 10 of theceramic substrate 2 as described above, to thereby provide the P/E unit18. The P/E layer 14 is suitably formed by a thick-film forming method,such as screen printing, spraying, coating or dipping. The thick-filmforming method utilizes a paste or slurry which contains as a maincomponent piezoelectric/electrostrictive ceramic particles havingaverage particle size of about 0.01 μm to 7 μm, preferably, 0.05 μm to 5μm, so as to form the film-like P/E layer 14 on the diaphragm portion 10of the ceramic substrate 2. In this case, the resultant film elementexhibits excellent piezoelectric/electrostrictive characteristics. Amongthe above-described thick-film forming methods, screen printing isparticularly favorably employed since it permits fine patterning at arelatively low cost. The thickness of the P/E layer 14 is preferably 50μm or smaller, more preferably in a range of 3 μm to 40 μm, to provide arelatively large displacement of the P/E layer 14 with a relatively lowvoltage.

The upper and lower electrodes 16, 12 of the P/E unit 18 are formed ofan electrically conductive material which can withstand oxidizingatmospheres having a considerably high temperature. For instance, theelectrodes 12, 16 may be formed of a single metal, an alloy of metals, amixture of a metal or alloy and an electrically insulating ceramic, oran electrically conductive ceramic. However, the electrode materialpreferably has a major component which consists of a noble metal havinga high melting point, such as platinum, palladium or rhodium, or analloy, such as silver-palladium alloy, silver-platinum alloy, orplatinum-palladium alloy. The electrodes 12, 16 may also be formed of acermet of platinum, and the ceramic material for the substrate 2 or thepiezoelectric/electrostrictive material for the P/E layer 14. Morepreferably, the electrodes 12, 16 are made solely of platinum, orincludes as a major component an alloy containing platinum. Where theabove-described cermets are used, the content of the substrate materialis preferably held within a range of 5-30% by volume, while thepiezoelectric/electrostrictive material is preferably held within arange of 5-20% by volume.

The electrodes 12, 16 are formed using the above-described conductivematerial, by suitably selected one of known film-forming methods whichinclude the thick-film forming methods as described above, and thin-filmforming methods, such as sputtering, ion-beam method, vacuum vapordeposition, ion plating, CVD and plating. The thick-film formingmethods, such as screen printing, spraying, dipping and coating, may befavorably employed for forming the lower electrode 12, while theabove-described thin-film forming methods as well as the thick-filmforming methods may be favorably employed for forming the upperelectrode 16. The thickness of the electrodes 12, 16 thus formed isgenerally not greater than 20 μm, preferably, not greater than 5 μm. Thetotal thickness of the P/E unit 18 which is the sum of the thickness ofthese electrodes 12, 16 and the P/E layer 14 is preferably 100 μm orsmaller, more preferably, 50 μm or smaller.

The piezoelectric/electrostrictive material for forming the P/E layer 14of the P/E unit 18 preferably contains as a major component leadzirconate titanate (PZT), lead magnesium niobate (PMN), lead nickelniobate (PNN), lead manganese niobate, lead antimony stannate, lead zincniobate, lead titanate, lead magnesium tantalate, lead nickel tantalate,or a mixture thereof. Further, a material (such as PLZT) containing anoxide or other compound of lanthanum, barium, niobium, zinc, cerium,cadmium, chromium, cobalt, antimony, iron, yttrium, tantalum, tungsten,nickel, manganese, lithium, strontium, or bismuth may be added as neededto the above-indicated piezoelectric/electrostrictive material.

Among the piezoelectric/electrostrictive materials as indicated above,it is recommended to use a material which includes as a major componentone of the following mixtures: a mixture of lead magnesium niobate, leadzirconate and lead titanate; a mixture of lead nickel niobate, leadmagnesium niobate, lead zirconate and lead titanate; a mixture of leadmagnesium niobate, lead nickel tantalate, lead zirconate and leadtitanate; and a mixture of lead magnesium tantalate, lead magnesiumniobate, lead zirconate and lead titanate. Further, these materials inwhich a part of lead (pb) is substituted by strontium and/or lanthanummay be favorably used for the P/E layer 14. Thesepiezoelectric/electrostrictive materials are recommended when the P/Elayer 14 is formed by a thick-film forming method, such as screenprinting. When the piezoelectric/electrostrictive material having threeor more components is used, its piezoelectric/electrostrictivecharacteristics vary depending upon the composition of the components ofthe material. However, a three-component material composed of leadmagnesium niobate, lead zirconate and lead titanate, or a four-componentmaterial composed of lead magnesium niobate, lead nickel tantalate, leadzirconate and lead titanate, or a four-component material composed oflead magnesium tantalate, lead magnesium niobate, lead zirconate andlead titanate preferably has a composition in the vicinity of phaseboundaries of a pseudo-cubic crystal phase, a tetragonal crystal phaseand a rhombohedral crystal phase. To assure sufficiently highpiezoelectric constant and electromechanical coupling factor, it isparticularly desirable to employ one of the following compositions, thatis, 1) a composition containing 15-50 mol % of lead magnesium niobate,10-45 mol % of lead zirconate and 30-45 mol % of lead titanate, 2) acomposition containing 15-50 mol % of lead magnesium niobate, 10-40 mol% of lead nickel tantalate, 10-45 mol % of lead zirconate and 30-45 mol% of lead titanate, 3) a composition containing 15-50 mol % of leadmagnesium niobate, 10-40 mol % of lead magnesium tantalate, 10-45 mol %of lead zirconate and 30-45 mol % of lead titanate.

The electrode films and P/E layer (12, 16, 14) formed on the outersurface of the diaphragm portion 10 of the ceramic substrate 2 asdescribed above may be either heat-treated (fired) in different stepsafter each of these films and layer is formed, for integration with thesubstrate 2, i.e., the diaphragm portion 10, or concurrentlyheat-treated (fired) in a single step for integration with the substrate2 after all of the films and layer are formed on the diaphragm portion10. Further, the above heat-treatment (firing) of the electrode films(12, 16) may not be required depending upon the method of forming thesefilms. The temperature of the heat treatment (firing) for integration ofthe electrode films and P/E layer with the diaphragm portion isgenerally controlled to be in a range of 500° C. to 1400° C., preferably1000° C. to 1400° C. To avoid changes in the composition of thepiezoelectric/electrostrictive material of the P/E layer 14 at a hightemperature, it is desirable to heat-treat or fire the P/E layer 14while controlling the firing atmosphere to include the evaporationsource of the piezoelectric/electrostrictive material. It is alsorecommended to fire the P/E layer 14 while it is covered with a suitablecovering member so that the surface of the layer 14 is not directlyexposed to the firing atmosphere. The covering member may be formed of amaterial similar to that of the ceramic substrate 2.

The P/E film element constructed as described above may be produced byvarious methods known to those skilled in the art, including thefollowing five methods. In the first method, a section of the diaphragmportion 10 on which the P/E unit 18 is not disposed is formed into adesired shape upon firing of the P/E layer 14 of the P/E unit 18, bycontrolling coefficients of thermal expansion of the ceramic substrate 2and P/E unit 18. In the second method, a pressing force is applied tothe diaphragm portion 10 when the P/E layer 14 of the P/E unit 18 isfired, so that the section of the diaphragm portion 10 on which the P/Eunit 18 is not disposed is formed into a desired shape. In the thirdmethod, the peripheral portion of the diaphragm portion 10 is formedinto a desired shape by controlling the firing shrinkage andsinterability of the P/E layer 14 of the P/E unit 18. In the fourthmethod, the diaphragm portion 10 in its green state is suitably shapedbefore firing thereof. In the fifth method, the width and thickness ofthe lower electrode 12 and P/E layer 14 are controlled. In the presentinvention, the P/E film element is advantageously produced by one of thefollowing methods illustrated shown in FIG. 8 through FIG. 12.

Referring to FIGS. 8(a)-8(c), there will be described one example of themethod for producing the P/E film element of FIG. 3. Initially, theceramic substrate 2 is prepared with the diaphragm portion 10 which hasa convex shape, that is, protrudes outwards in the direction away fromthe window 6 by an amount "h" as indicated in FIG. 8(a). After theceramic substrate 2 is fired, the lower electrode 12 and the P/E layer14 are successively laminated on the outer surface of the convexdiaphragm portion 10 by a suitable film-forming method, such that thelaminar structure of the electrode 12 and P/E layer 14 is spaced apartfrom the opposite ends of the window 6 as seen in FIG. 8(b), bypredetermined suitable distances n, n' as indicated in FIG. 8(b). Whenthe P/E layer 14 is fired after the upper electrode 16 is formed thereonas needed, the P/E layer 14 has greater firing shrinkage than thediaphragm portion 10 which has already been fired, whereby a centralpart of the diaphragm portion 10 which carries the P/E unit 18 is curvedor deflected inwards, toward the window 6, as shown in FIG. 8(c).Accordingly, there are formed two stress releasing sections 20, 20 eachhaving an upwardly curved convex shape, on the opposite sides of the P/Eunit 18, respectively.

The ceramic substrate 2 in the processes of FIGS. 8(a) and 8(b), whichhas the convex diaphragm portion 10 that protrudes outwards, is easilyobtained by controlling the sintering rate or speed or the shrinkagepercentage of the base plate 4 and diaphragm plate 8 (FIGS. 1 and 2),suitably adjusting the configuration of the diaphragm plate 8 in itsgreen state before firing thereof, or utilizing the difference in thethermal expansion coefficient between the two plates 4, 8. Morespecifically described, the diaphragm portion 10 protrudes outwards ifthe sintering of a green sheet which gives the diaphragm plate 8precedes the sintering of a green sheet which gives the base plate 4, orif the shrinkage caused by the sintering of the green sheet for the baseplate 4 is greater than that of the green sheet for the diaphragm plate8.

The amount "h" of protrusion of the diaphragm portion 10 of the ceramicsubstrate 2 is generally 1-20% of the shortest dimension (m) as measuredalong the straight line which passes the center of the window,preferably 2-10% of the dimension (m). If the protrusion amount "h" istoo small, the amount of downward deflection of the diaphragm portion 10upon firing of the P/E layer 14 may be excessively large. If theprotrusion amount "h" is too large, it may be difficult for thediaphragm portion 10 to be downwardly deflected into the window 6 uponfiring of the P/E layer 14.

Next, there will be described a method of producing the P/E film elementof FIG. 4 with reference to FIGS. 9(a)-9(c). Initially, there isprepared a ceramic substrate 2 whose diaphragm portion 10 has a convexshape similar to that of FIG. 8(a). On the outer surface of the convexdiaphragm portion 10, the P/E unit 18 is formed such that the P/E unit18 is partly disposed over one of the opposite portions of the peripheryof the window 6, as shown in FIG. 9(b). Then, the P/E layer 14 is firedso as to provide a P/E film element which has a stress releasing section20 formed along and adjacent to the other of the opposite peripheralportions of the window 6 as shown in FIG. 9(c). The part of thediaphragm portion 10 on which the P/E unit 18 is formed is downwardlydeflected on the side of the window 6, assuming a concave shape, due tostresses which occur due to shrinkage of the P/E layer 14 upon itsfiring. On the other hand, the part of the diaphragm portion 10 whichdoes not carry the P/E unit 18 maintains the convex shape after firing,thus providing the upwardly curved convex stress releasing section 20,as shown in FIG. 9(c).

Referring next to FIGS. 10(a)-10(c), there is shown a method ofproducing the P/E film element of FIG. 5. Initially, there is prepared aceramic substrate 2 whose diaphragm portion 10 has a corrugated shape asshown in FIG. 10(a). The ceramic substrate 2 having the corrugateddiaphragm portion 10 can be easily obtained by the methods as describedabove. For instance, the diaphragm portion 10 is formed into thecorrugated shape by sintering of a green sheet for the base plate 4before sintering of a green sheet for the diaphragm plate 8. On the thusprepared ceramic substrate 2, the P/E unit 18 is formed as shown in FIG.10(b). Then, the P/E layer 14 of the P/E unit 18 is fired so as toprovide the P/E film element as shown in FIG. 10(c), wherein a centralpart of the diaphragm portion 10 which bears the P/E unit 18 isdownwardly deflected on the side of the window 6, to form a concaveshape, while the end parts of the diaphragm portion 10 which are locatedon the opposite sides of the P/E unit 18 as seen in FIG. 10(c) maintainthe corrugated shape after the firing, thus providing two stressreleasing sections 20, 20 as shown in FIG. 10(c). The thus obtained P/Efilm element assures improved displacement amount of the diaphragmportion 10 upon actuation of the P/E unit 18. Further, the present filmelement does not suffer from the adverse influence which would occurwhen the adjacent two or more P/E units are concurrently actuated, tothereby prevent considerable reduction in the amounts of displacement ofthe P/E units.

Referring to FIGS. 11(a)-11(c), there will be described one example of amethod of producing the P/E film element of FIGS. 6 and 7. Initially,there is prepared a ceramic substrate 2 as shown in FIG. 11(a), whereinthe diaphragm portion 10 has two recessed portions formed adjacent tothe opposite peripheral portions of the window 6 which give the shortestdimension (m) of the window 6. The two recessed portions of thediaphragm portion 10, which have a suitable depth will provide twostress releasing sections. The ceramic substrate 2 having the tworecessed portions is obtained according to a method as illustrated inFIGS. 12, for instance.

Described more specifically, referring to FIGS. 12(a) and 12(b), thereis initially prepared a ceramic substrate 2 having a convex diaphragmportion 10 which protrudes outwards in the direction away from thewindow 6, as in the method of FIGS. 8. As shown in FIG. 12(a), a pair ofspacers 19, 19 formed of a ceramic material and having a predeterminedheight are disposed along the opposite peripheral edges of the window 6,respectively, such that the window 6 is located between the two spacers19. Then, a suitable pressing jig or pressure member 21 is pressedagainst the curved convex surface of the diaphragm portion 10 while thediaphragm portion 10 is heated and held at a suitable temperature, sothat the end parts of the convex surface of the diaphragm portion 10 aredownwardly deformed or deflected into the window, whereby the sectionsof the diaphragm portion 10 adjacent to the opposite peripheral edges ofthe window 6 are downwardly curved into the window 6 as shown in FIG.12(b). The pressing jig 21 is pressed against the convex surface of thediaphragm portion 10 until the jig 21 is brought into abutting contactwith the spacers 19. Thus, the diaphragm portion 10 is given the tworecessed portions adjacent to the edges of the window 6, as describedabove. On the other hand, the upwardly projecting central part of thediaphragm portion 10 is shaped following the profile of the abuttingsurface of the jig. 21. In this specific example, the central part ofthe diaphragm portion 10 is substantially flattened. As a result, theamount of protrusion of the upwardly projecting central part is madesmaller than that of the original shape of FIG. 11(a). The spacers 19may be formed integrally with the diaphragm plate 8 and left on theceramic substrate 2, or may be removed therefrom as needed after thepressing process of the diaphragm portion 10.

On a central part of the thus prepared ceramic substrate 2 whosediaphragm portion 10 has the two recessed portions formed as describedabove, the lower electrode 12 and P/E layer 14 are successivelylaminated by a suitable film-forming method, such that the laminarstructure of the lower electrode 12 and P/E layer 14 is spaced apartfrom the opposite ends of the window 6, by suitable distances n, n',respectively, as shown in. FIG. 11(b). When the P/E layer 14 is firedafter the upper electrode 16 is formed as needed, the P/E layer 14 hasgreater firing shrinkage than the diaphragm portion 10 which has beenfired, whereby the central part of the diaphragm portion 10 isdownwardly deformed toward the window 6, as shown in FIG. 11(c).According to this method, two stress releasing sections 20, 20 eachhaving a concave shape are formed on the opposite sides of the P/E unit18, respectively. Although, in the present method, the central part ofthe diaphragm portion 10 which carries the P/E unit 18 is downwardlycurved as shown in FIG. 11(c) as in the P/E film element of FIG. 6, thecentral part of the diaphragm portion 10 may be upwardly curved as inthe film element of FIG. 7. In this case, the diaphragm portion 10 isformed into the convex shape by controlling the degree of deformation ofthe central part of the diaphragm portion 10, for example, by reducingthe firing shrinkage of the P/E layer 14 (to a value lower than thatused in the embodiment of FIG. 11).

In the piezoelectric/electrostrictive film element constructed accordingto the present method, stresses caused by firing shrinkage of the P/Elayer 14 which is in contact with the diaphragm portion 10 of theceramic substrate 2 during the heat treatment of the P/E layer 14 can bereduced owing to the presence of the stress releasing section orsections 20, whereby the P/E layer 14 is sufficiently densified, and theresidual stresses may be reduced.

In the above-described method, the lower electrode film 12, P/E layer 14and upper electrode film 16 are formed on the outer surface of thediaphragm portion 10 of the ceramic substrate 2, by the above-indicatedfilm-forming method(s), and then fired at the above-indicated firingtemperature, to thereby provide respective films and layer havingdesired thickness values. Thus, the P/E unit 18 is formed integrally onthe appropriate portion of the diaphragm portion 10. While the P/E layer14 is desirably fired just after it is formed on the lower electrode 12,that is, before the upper electrode 16 is formed, the firing of the P/Elayer 14 may be effected after the upper electrode 16 is formed on theP/E layer 14.

In the P/E film element thus obtained according to the presentinvention, the diaphragm portion 10 has the stress releasing section 20formed on at least one of the opposite sides of the P/E unit 18, makingit possible to convert stresses generated in the P/E unit 18 intodisplacement of the diaphragm portion 10 with high efficiency. Further,when a plurality of P/E units 18 are provided on respective diaphragmportions 10 of the ceramic substrate 2, the amount of displacement ofeach diaphragm portion 10 which occurs when the two or more adjacent P/Eunits 18 are simultaneously actuated is almost equal to the amount ofdisplacement which occurs when only one P/E unit 18 is actuated.Accordingly, the P/E film element may be advantageously used as sensors,actuators, or transducers, for example.

The present P/E film element, which effectively undergoes displacementupon actuation of the P/E unit 18. formed on the outer surface of thediaphragm portion 10, is advantageously used as filters, varioussensors, such as an acceleration sensor, a shock sensor, an ultrasonicsensor or an angular velocity sensor, transformers, microphones,sounding bodies, such as a loudspeaker, discriminators and variousvibrators and resonators for power devices and communication devices.Further, the present film element may be particularly advantageouslyused as a uni-morph, bi-morph or other type ofpiezoelectric/electrostrictive actuators which produce displacement inthe form of bending or deflection, and are used for display devices,servo-displacement elements, pulse-driven motors, ultrasonic motors,piezoelectric fans and others, which elements and motors are describedin "FUNDAMENTALS TO APPLICATIONS OF PIEZOELECTRIC/ELECTROSTRICTIVEACTUATORS", Kenji Uchino, Japan Industrial Technology Center, publishedby Morikita-shuppan.

Referring next to FIG. 13 schematically showing an example of a P/E filmelement according to the present invention, and to FIG. 14 schematicallyshowing the film element in cross section taken along line 14--14 ofFIG. 13. The present P/E film element has an integral structure whichincludes a ceramic substrate 22 and a plurality ofpiezoelectric/electrostrictive units (hereinafter referred to as "P/Eunits") 24 formed on relevant outer surfaces of thin-walled diaphragm orvibratile portions of the ceramic substrate 22. In operation, each ofthe diaphragm portions of the ceramic substrate 22 is flexed, bent,deflected or otherwise deformed upon application of a voltage to thecorresponding P/E unit 24.

More specifically, the ceramic substrate 22 has an integral laminarstructure which consists of a relatively thin closure plate (diaphragmplate) 26, a connecting plate (base plate) 28, and a spacer plate (baseplate) 30 interposed between the closure and connecting plates 26, 28.These plates 26, 28, 30 are formed of a zirconia material. Theconnecting plate 28 has three communication holes 32, which are formedthrough the thickness of the plate 28 with a suitable spacingtherebetween. The number, shape, dimensions, position and otherparameters of the communication holes 32 may be suitably determined,depending upon a specific application of the film element. The spacerplate 30 is formed with a plurality of square windows 36 (three in thisembodiment). This spacer plate 30 is superposed on the connecting plate28 such that the communication holes 32 of the connecting plate 28communicate with the respective windows 36. The closure plate 26 issuperposed on one major surface of the spacer plate 30 remote from theconnecting plate 28, so as to close the openings of the windows 36 ofthe spacer plate 30. With the closure plate 26, spacer plate 30 andconnecting plate 28 thus superposed on each other, three pressurechambers 38 are formed within the ceramic substrate 22, such that thechambers 38 communicate with an exterior space through the communicationholes 32.

The ceramic substrate 22 is an integral fired body formed of a suitableceramic material, such as a zirconia material, as described above. Whilethe ceramic substrate 22 of the present embodiment is a three-layerstructure consisting of the closure plate 26 (diaphragm plate), spacerplate 30 (base plate) and connecting plate 28 (base plate), thesubstrate may be formed as a four-layer or other multi-layer integralstructure having four or more layers or plates, as shown in FIG. 15.Described more specifically, the film element of FIG. 15 has afive-layer structure, wherein two spacer plates 30 and two connectingplates 28 are alternately superposed on each other, on the major surfaceof the closure plate 26 remote from the P/E units 24. In the thus formedfive-layer integral structure of the film element, the pressure chambers38 and intermediate chambers 46 formed within the ceramic substratecommunicate with each other through communication holes 48 formedthrough the thickness of the connecting plates 28.

Film-like P/E units 24 are formed on the outer surface of the closureplate 26, such that the P/E units 24 are aligned with the respectivepressure chambers 38 as viewed in a plane parallel to the closure plate26. Each of the P/E units 24 includes a lower electrode 40, apiezoelectric/electrostrictive layer (hereinafter referred to as "P/Elayer") 42 and an upper electrode 44 which are successively formed by asuitable film-forming method(s) on a portion of the closure plate 26which is located in alignment with one of the windows 36 of the ceramicsubstrate 22, that is, on the outer surface of one diaphragm portion ofthe ceramic substrate. In operation, the pressure in the pressurechamber 38 is increased upon actuation of the corresponding P/E unit 24,so that a fluid contained in the pressure chamber 38 can be effectivelydischarged through the corresponding communication hole 48. The P/E filmelement thus constructed may be used not only as an actuator but also asa sensor or the like, which is adapted to generate a voltage signal thatrepresents flexural displacement of the diaphragm portion of the ceramicsubstrate.

In the above-constructed P/E film element, the stress releasing sectionsare formed on the opposite sides of each of the P/E unit 24, as viewedin the direction in which the windows 36 are arranged in a straight row.

While the P/E film element according to the present invention may beused as actuators, sensors, and transducers, particularly advantageouslyas a member of display devices, loudspeakers, servo-displacementelements, pulse-driven motors, ultrasonic motors, acceleration sensors,shock sensors, oscillators, vibrators and resonators, it is to beunderstood that the present film element has other applications known inthe art.

EXAMPLES

To further clarify the present invention, some examples of the P/E filmelements of the present invention will be described. However, it is tobe understood that the present invention is by no means limited to thedetails of the following examples, but may be embodied with variouschanges, modifications and improvements which may occur to those skilledin the art, without departing from the principle and scope of thepresent invention defined in the attached claims.

EXAMPLE 1

Initially, there was prepared a rectangular ceramic substrate havingfour rectangular windows each of which had a width of 0.5 mm and alength of 0.7 mm. The four rectangular windows are arranged in astraight row in the longitudinal direction of the ceramic substrate suchthat the 0.7 mm-long long sides of the adjacent windows are spaced 0.2mm apart from each other. The windows were closed by respective 10μm-thick diaphragm portions of the substrate. The diaphragm portions andbase portion of the ceramic substrate were formed of a powder ofzirconia partially stabilized by yttria, which has an average particlesize of 0.4 μm. The partially stabilized zirconia was formed into greensheets, which were then fired by a known method. The base portion of thesubstrate has a thickness of 200 μm when measured after firing of thesubstrate.

On an outer surface of each of the diaphragm portions of the ceramicsubstrate, a layer of a platinum paste was formed by screen printing,dried at 120° C. for ten minutes, and fired at 1350° C. for two hours,to provide an lower electrode having a thickness of 5 μm. Then, apiezoelectric/electrostrictive layer (hereinafter referred to as "P/Elayer") was formed on the lower electrode by using a piezoelectric orelectrostrictive material consisting essentially of lead magnesiumniobate, lead zirconate and lead titanate. This material was applied bythe screen printing, dried at 120° C. for twenty minutes, and fired at1300° C., to provide the P/E layer having a thickness of 30 μm. The P/Elayer was formed on the lower electrode such that each of the oppositeend faces of the P/E layer was spaced apart from the corresponding oneof the opposite ends of the window (0.7 mm length side of the window 6),by a a distance of 0.1 mm. A suitable mold formed of alumina ceramic wasapplied to the ceramic substrate with the lower electrode and P/E layerthus formed on the respective diaphragm portions, such that each of thediaphragm portions was sandwiched on its opposite sides by the aluminaceramic mold. In this state, the ceramic substrate was re-fired at 1300°C. Thus, the ceramic substrate having the "M"-shaped diaphragm portionsas shown in FIG. 3 was obtained, wherein two stress releasing sectionshaving an upwardly curved convex shape protruding away from the window 6were formed on the opposite sides of each P/E layer, respectively,namely, at the 0.1 mm-wide opposite end parts of the diaphragm portion10, each of which is located between one of the opposite ends or edgesof the window 6 and the corresponding one of the opposite end faces ofthe P/E layer.

Thereafter, a Cr thin film and Cu thin film were formed by sputtering onthe P/E layer provided on each of the "M" shaped diaphragm portions ofthe ceramic substrate, so as to provide an upper electrode having atotal thickness of 0.3 μm. In this manner, the intended P/E film elementaccording to the present invention was obtained. The thus obtained filmelement was subjected to polarization by applying 100V between the upperand lower electrodes of the P/E unit, so that the P/E layers are to bedisplaced in a direction toward the corresponding diaphragm portionsupon application of a voltage to the P/E unit during use of the filmelement.

As a comparative example, a conventional P/E film element was producedby using the ceramic substrate prepared as described above, and bondinga 30 μm-thick plate made of a piezoelectric or electrostrictive materialto each diaphragm portion of the substrate, with a conductive adhesive.

To evaluate piezoelectric/electrostrictive characteristics of thepresent and conventional film elements obtained as described above, avoltage of 30V was applied between the upper and lower electrodes ofeach of the P/E units of each film element, in the direction of thepolarization treatment explained above, and the amount of displacementof the relevant P/E unit was measured by a laser Doppler device. In thismanner, all of the four P/E units were actuated at different times, anda first average of the displacement amounts of these P/E units wascalculated. Similarly, a voltage of 30V was applied to all of the fourP/E units of each element for simultaneous actuation thereof, and theamounts of displacement of the respective P/E units were measured. Asecond average of the amounts of displacement of the four P/E units thatwere actuated at the same time was also calculated. Based on the thusobtained results, there was calculated the ratio of the displacementamount measured upon actuation of all the P/E units (simultaneousactuation) to the displacement amount measured upon actuation of eachsingle P/E unit (single actuation), as represented by (the secondaverage/the first average)×100 (%) The results of the measurement andcalculation are indicated in the TABLE 1 below.

                  TABLE 1                                                         ______________________________________                                                   Simultaneous actuation/                                                                    Single                                                           single actuation                                                                           actuation                                             ______________________________________                                        present      100%           0.23 μm                                        film element                                                                  conventional  50%           0.11 μm                                        film element                                                                  ______________________________________                                    

It will be apparent from the above results of TABLE 1 that the P/E filmelement according to the present invention, in which each of thediaphragm portions of the ceramic substrate has the stress releasingsections which have an upwardly curved convex shape and which are formedat the opposite end parts of the diaphragm portion on which the P/E unitis not formed, exhibited substantially the same amount of displacementin the case where all the P/E units were simultaneously actuated, and inthe case where each one P/E unit was actuated alone. This is a greatimprovement over the conventional film element, in which the ratio ofthe displacement amount upon actuation of all the P/E units to thedisplacement amount upon actuation of each single P/E unit was 50%.

EXAMPLE 2

Initially, a ceramic powder, which has an average particle size of 0.4μm and consists essentially of 85% by weight of a zirconia powderpartially stabilized by 3 mol % of yttria, and 15% by weight of alumina,was mixed by a known method with a binder, a plasticizer and an organicsolvent, so as to prepare a slurry. This slurry was used to form, bydoctor blade method, a green sheet which provides after firing the baseplate of the ceramic substrate having a thickness of 200 μm.

On the other hand, a zirconia powder partially stabilized by 3 mol % ofyttria and having the average particle size of 0.3 μm was mixed by aknown method with a binder, a plasticizer and an organic solvent, so asto prepare a slurry. This slurry was used to form, by a reverse rollcoater machine, a green sheet which provides after firing the diaphragmplate having a thickness of 10 μm.

Thereafter, the green sheet for the base plate was punched in a patternby means of a suitable metal mold, so as to form the windows. Then, thegreen sheet for the diaphragm plate was superposed on the green sheetfor the base plate, and bonded together by thermo-compression underpressure of 100 kg/cm², at 80° C. for one minute. The thus obtainedintegral laminar structure was fired at 1500° C. for two hours, tothereby provide the ceramic substrate with the convex diaphragm portionsprotruding outwards by a distance (h) of 20 μm.

Subsequently, the lower electrode and P/E layer were formed on the outersurface of each of the convex diaphragm portions of the thus obtainedceramic substrate, in the same manner as in EXAMPLE 1. After firing ofthe lower electrode and P/E layer in the same manner as in EXAMPLE 1,the ceramic substrate with the "M"-shaped diaphragm portions as shown inFIG. 3 was obtained. Described more specifically, in the obtainedceramic substrate, the central part of the diaphragm portion on whichthe lower electrode and P/E layer were formed was downwardly curved toprovide a concave shape. On the other hand, the opposite end parts ofthe diaphragm portion which do not carry the lower electrode and P/Elayer maintained the original convex shape protruding outwards away fromthe window, to thereby provide two stress releasing sections each havingan upwardly curved convex shape, on the opposite sides of the P/E layer,respectively. Then, the upper electrode was formed on the fired P/Elayer of each P/E unit in the same manner as in EXAMPLE 1, so as toprovide the P/E film element constructed according to the presentinvention.

The thus obtained ceramic substrate was subjected to polarizationtreatment. Then the first average of displacement amounts of all of theP/E units measured when they were actuated at different times, and thesecond average of displacement amounts of all of the P/E units measuredwhen they were actuated simultaneously were obtained in the same manneras in EXAMPLE 1. Based on the thus obtained results, the ratio asrepresented by (the second average/the first average)×100 (%) wascalculated. The results showed that the present film element exhibitedexcellent characteristics similar to those of the film element ofEXAMPLE 1.

EXAMPLE 3

Initially, a powder, which has an average particle size of 0.4 μm andconsists essentially of 99.75% by weight of a zirconia powder partiallystabilized by 3 mol % of yttria, and 0.25% by weight of alumina, washeat-treated at 500° C. for two hours, and then was mixed in a ball millfor thirty hours by an ordinary method with a binder, a plasticizer andan organic solvent, so as to prepare a slurry. The obtained slurry wasused to form, by doctor blade method, a green sheet which provides afterfiring the base plate of the ceramic substrate having a thickness of 200μm.

In the meantime, a powder, which has an average particle size of 0.2 μmand consists essentially of 70% by weight of a zirconia powder partiallystabilized by 3 mol % of yttria, and 30% by weight of alumina, was mixedin a ball mill for thirty hours by an ordinary method with a binder, aplasticizer and an organic solvent, so as to prepare a slurry. Theslurry was used to form, by a reverse roll coater machine, a green sheetwhich provides after firing the diaphragm plate having a thickness of 10μm.

Thereafter, the green sheet for the base plate was punched in a patternby means of a suitable metal mold, so as to form the windows. Then, thegreen sheet for the diaphragm plate was superposed on the green sheetfor the base plate, and was bonded together by thermo-compression underpressure of 100 kg/cm² at 80° C. for one minute. The thus obtainedintegral laminar structure was fired at 1500° C. for two hours, toprovide the ceramic substrate with diaphragm portions having acorrugated shape as shown in FIG. 10(a).

Subsequently, the lower electrode and P/E layer were formed on the outersurface of each of the corrugated diaphragm portions of the thusobtained ceramic substrate, in the same manner as in EXAMPLE 1. Afterfiring of the lower electrode and P/E layer, there was obtained aceramic substrate with diaphragm portions each having corrugated stressreleasing sections formed on the opposite sides of the P/E layer,respectively. Namely, the corrugated shape of the diaphragm portion wasmaintained at the opposite end parts of the diaphragm portion on whichthe lower electrode and P/E layer are not disposed, so as to provide thecorrugated stress releasing sections. Then, the upper electrode wasformed on the fired P/E layer in the same manner as in EXAMPLE 1,whereby a P/E film element constructed according to the presentinvention was produced.

The thus obtained film element was subjected to polarization treatmentand then, the ratio of the displacement amount upon actuation of all theP/E units (simultaneous actuation) to the displacement amount uponactuation of a single P/E unit (single actuation) was obtained in thesame manner as in EXAMPLE 1. The results showed that the above-indicatedratio was 85% in the present film element having the corrugated stressreleasing sections formed on the opposite sides of each of the P/Eunits. Thus, it was recognized that the corrugated stress releasingsections formed as described above were effective to preventconventionally experienced considerable reduction in the amount ofdisplacement when the two or more adjacent P/E units were actuated atthe same time. The results also showed that the average value of theamounts of displacement of all P/E units measured when they wereactuated at different times (single actuation) was 0.18 μm.

EXAMPLE 4

In the same manner as in EXAMPLE 3, a green sheet for the base plate wasformed by using a powder having an average particle size of 0.4 μm andconsisting essentially of 99.9% by weight of a zirconia powder partiallystabilized by 4 mol % of yttria, and 0.1 % by weight of alumina.Similarly, a green sheet for the diaphragm plate was formed by using apowder having an average particle size of 0.4 μm and consistingessentially of 99.5% by weight of a zirconia powder partially stabilizedby 4 mol % of yttria, and 0.5 % by weight of alumina.

The thus obtained green sheet for the base plate was punched in apattern by using a suitable metal mold, so as to form the windows. Thegreen sheet for the diaphragm plate was superposed on the green sheetfor the base plate, and thermo-compressed under pressure of 100 kg/cm²at 80° C. for one minute. The thus obtained integral laminar structurewas fired at 1500° C. for two hours, to thereby provide a ceramicsubstrate with convex diaphragm portions each of which protrudesoutwards by a distance of 50 μm.

On the above obtained ceramic substrate, a pair of alumina spacershaving a height of 20 μm were disposed along the long sides of each ofthe windows such that the spacers sandwich the opening of each window,as shown in FIG. 12(a). Then, a planar porous pressing jig made ofalumina was disposed on each of the convex diaphragm portions. In thisstate, the ceramic substrate was fired at 1500° C. for five hours, so asto provide a ceramic substrate with each diaphragm portion having aconvex central part having a height of 20 μm and concave end partshaving a depth of 15 μm formed adjacent to the opposite edges of theabove-indicated long sides of the window, respectively. Then, as shownin FIG. 11(b), the lower electrode and P/E layer were formed on theconvex central part of each of the diaphragm portions, in the samemanner as in EXAMPLE 1. By firing of the lower electrode and P/E layer,the convex central part of the diaphragm portion on which the lowerelectrode and P/E layer were formed was downwardly curved toward thewindow, while the concave parts of the diaphragm portion on which thelower electrode and P/E layer were not formed maintained the originalconcave shape, so as to function as the stress releasing sections eachhaving a curved concave shape. The upper electrode was formed on each ofthe fired P/E layers, whereby a P/E film element constructed accordingto the present invention was produced.

The thus obtained film element was subjected to polarization treatmentas in EXAMPLE 1, and the ratio of the displacement amount upon actuationof all the P/E units (simultaneous actuation) to the displacement amountupon actuation of a single P/E unit (single actuation) was obtained asin EXAMPLE 1. The results showed that the above-indicated ratio was 90%in the present film element with the diaphragm portions each of whichhas the concave stress releasing sections formed on the opposite sidesof the corresponding P/E unit. Thus, it was recognized that the stressreleasing sections having the concave shape formed as described abovewere effective to prevent the considerable reduction in the amount ofdisplacement conventionally experienced when the two or more P/E unitsare actuated at the same time. The results also showed that the averagevalue of the displacement amounts measured when all the P/E units wereactuated at different times was 0.20 μm.

What is claimed is:
 1. A piezoelectric/electrostrictive film element,comprising:a ceramic substrate including a base portion having at leastone window, and a diaphragm portion formed as an integral part of saidbase portion and closing each of said at least one window; a film-likepiezoelectric/electrostrictive unit including a lower electrode, apiezoelectric/electrostrictive layer and an upper electrode, which areformed in the order of description on an outer surface of said diaphragmportion by a film-forming process; said piezoelectric/electrostrictiveunit being disposed on said diaphragm Portion such that at least one ofopposite end faces of said piezoelectric/electrostrictive unit is spacedapart from a corresponding one of opposite portions of a periphery ofsaid each window in a direction toward a center of said diaphragmportion; a stress releasing section constituted by each of at least oneof opposite end parts of said diaphragm portion which is located betweensaid at least one of the opposite end faces of saidpiezoelectric/electrostrictive unit and the corresponding at least oneof the opposite portions of the periphery of said each window, saidstress releasing section being curved with respect to a plane whichincludes a major surface of said base portion at which said each windowis closed by said diaphragm portion, wherein said opposite portions ofsaid periphery of said each window are opposed to each other in adirection parallel to a straight line which is parallel to said plane ofsaid major surface of said base portion and passes through a center ofsaid each window and along which said each window has a shortestdimension, a dimension of said stress releasing section as measured insaid direction being not greater than 40% of said shortest dimension ofsaid each window.
 2. A piezoelectric/electrostrictive film element,comprising:a ceramic substrate including a base portion having at leastone window, and a diaphragm portion formed as an integral part of saidbase portion and closing each of said at least one window; a film-likepiezoelectric/electrostrictive unit including a lower electrode, apiezoelectric/electrostrictive layer and an upper electrode, which areformed in the order of description on an outer surface of said diaphragmportion by a film-forming process; said piezoelectric/electrostrictiveunit being disposed on said diaphragm portion such that at least one ofopposite end faces of said piezoelectric/electrostrictive unit is spacedapart from a corresponding one of opposite portions of a periphery ofsaid each window in a direction toward a center of said diaphragmportion; a stress releasing section constituted by each of at least oneof opposite end Parts of said diaphragm portion which is located betweensaid at least one of the opposite end faces of saidpiezoelectric/electrostrictive unit and the corresponding at least oneof the opposite portions of the periphery of said each window, saidstress releasing section being curved with respect to a plane whichincludes a major surface of said base portion at which said each windowis closed by said diaphragm portion, wherein saidpiezoelectric/electrostrictive unit is disposed such that only one ofsaid opposite end faces of said piezoelectric/electrostrictive unit isspaced apart from the corresponding one of said opposite portions of theperiphery of said each window in the direction toward the center of saiddiaphragm portion, said stress releasing section being provided at onlyone of said opposite end parts of said diaphragm portion located betweensaid one of the opposite end faces of saidpiezoelectric/electrostrictive unit and said corresponding one of saidopposite portions of said periphery of said each window.
 3. Apiezoelectric/electrostrictive film element, comprising:a ceramicsubstrate including a base portion having at least one window, and adiaphragm portion formed as an integral part of said base portion andclosing each of said at least one window; a film-likepiezoelectric/electrostrictive unit including a lower electrode, apiezoelectric/electrostrictive layer and an upper electrode, which areformed in the order of description on an outer surface of said diaphragmportion by a film-forming process; said piezoelectric/electrostrictiveunit being disposed on said diaphragm portion such that at least one ofopposite end faces of said piezoelectric/electrostrictive unit is spacedapart from a corresponding one of opposite portions of a periphery ofsaid each window in a direction toward a center of said diaphragmportion; a stress releasing section constituted by each of at least oneof opposite end parts of said diaphragm portion which is located betweensaid at least one of the opposite end faces of saidpiezoelectric/electrostrictive unit and the corresponding at least oneof the opposite portions of the periphery of said each window, saidstress releasing section being curved with respect to a plane whichincludes a major surface of said base portion at which said each windowis closed by said diaphragm portion, wherein saidpiezoelectric/electrostrictive unit is disposed such that both of saidopposite end faces of said piezoelectric/electrostrictive unit arespaced apart from the corresponding opposite portions of the peripheryof said each window in the direction toward the center of said diaphragmportion, said stress releasing section being provided at each of saidopposite end parts of said diaphragm portion located between saidopposite end faces of said piezoelectric/electrostrictive unit and thecorresponding opposite portions of the periphery of said each window,and said stress releasing section provided at each of said opposite endparts of said diaphragm portion has an upwardly convex shape, protrudingfrom said plane in a direction away from said each window, saiddiaphragm portion including a central part on which saidpiezoelectric/electrostrictive unit is formed, said central part havinga downwardly concave shape, protruding from said plane in a directiontoward said each window.
 4. A piezoelectric/electrostrictive filmelement according to claim 2, wherein said stress releasing sectionprovided at only one of said opposite end parts of said diaphragmportion has an upwardly convex shape, protruding from said plane in adirection away from said each window, a part of said diaphragm portionwhich includes the other of said opposite end parts and on which saidpiezoelectric/electrostrictive unit is formed having a downwardlyconcave shape, protruding from said plane in a direction toward saideach window.
 5. A piezoelectric/electrostrictive film element,comprising:a ceramic substrate including a base portion having at leastone window, and a diaphragm portion formed as an integral part of saidbase portion and closing each of said at least one window; a film-likepiezoelectric/electrostrictive unit including a lower electrode, apiezoelectric/electrostrictive layer and an upper electrode, which areformed in the order of description on an outer surface of said diaphragmportion by a film-forming process; said piezoelectric/electrostrictiveunit being disposed on said diaphragm portion such that at least one ofopposite end faces of said piezoelectric/electrostrictive unit is spacedapart from a corresponding one of opposite portions of a periphery ofsaid each window in a direction toward a center of said diaphragmportion; a stress releasing section constituted by each of at least oneof opposite end parts of said diaphragm portion which is located betweensaid at least one of the opposite end faces of saidpiezoelectric/electrostrictive unit and the corresponding at least oneof the opposite portions of the periphery of said each window, saidstress releasing section being curved with respect to a plane whichincludes a major surface of said base portion at which said each windowis closed by said diaphragm portion, wherein said stress releasingsection has an upwardly convex shape, protruding from said plane in adirection away from said each window, and said ceramic substratecomprises a diaphragm plate which includes said diaphragm portion andwhich is formed on said major surface of said base portion such thatsaid diaphragm portion closes said each window, said upwardly convexshape of said stress releasing section having a height not greater thantwice a thickness of said diaphragm portion, said height being measuredfrom one of opposite major surfaces of said diaphragm plate which isremote from said base portion.
 6. A piezoelectric/electrostrictive filmelement, comprising:a ceramic substrate including a base portion havingat least one window, and a diaphragm portion formed as an integral partof said base portion and closing each of said at least one window; afilm-like piezoelectric/electrostrictive unit including a lowerelectrode, a piezoelectric/electrostrictive layer and an upperelectrode, which are formed in the order of description on an outersurface of said diaphragm portion by a film-forming process: saidpiezoelectric/electrostrictive unit being disposed on said diaphragmportion such that at least one of opposite end faces of saidpiezoelectric/electrostrictive unit is spaced apart from a correspondingone of opposite portions of a periphery of said each window in adirection toward a center of said diaphragm portion; a stress releasingsection constituted by each of at least one of opposite end parts ofsaid diaphragm portion which is located between said at least one of theopposite end faces of said piezoelectric/electrostrictive unit and thecorresponding at least one of the opposite portions of the periphery ofsaid each window, said stress releasing section being curved withrespect to a plane which includes a major surface of said base portionat which said each window is closed by said diaphragm portion, whereinsaid stress releasing section has a downwardly concave share, protrudingin a direction toward said each window, and said ceramic substratecomprises a diaphragm plate which includes said diaphragm portion andwhich is formed on said major surface of said base portion such thatsaid diaphragm portion closes said each window, said downwardly concaveshape of said stress releasing section having a height not greater thantwice a thickness of said diaphragm portion, said height being measuredfrom one of opposite major surfaces of said diaphragm plate which isremote from said base portion.